1. Field of the Invention
The present invention relates to a recording information reproducing apparatus for reproducing recording information from a recording medium.
2. Description of Related Art
FIG. 1 is a diagram showing the structure of a conventional recording information reproducing apparatus.
In FIG. 1, a pickup 1 reads recording information from a recording disc 3 which is rotated by a spindle motor 2 and supplies a read signal obtained by the reading to an A/D converter 5. The A/D converter 5 sequentially samples the read signal at a timing of a clock signal supplied from a PLL circuit 10 and supplies a read sampling value sequence obtained by the reading to each of the PLL circuit 10 and a decoding circuit 30.
A phase error detecting circuit 11 in the PLL circuit 10 detects a phase error which develops in the read signal based on the read sampling value sequence sequentially supplied from the A/D converter 5 and supplies a phase error signal corresponding to the amount of the phase error to an LPF (low pass filter) 12. The LPF 12 supplies an average phase error signal obtained by averaging the phase error signals to a frequency variable oscillator 13. The frequency variable osillator 13 generates a clock signal having a frequency according to the average phase error signal and supplies it to the A/D converter 5. The decoding circuit 30 is constituted, for example, by a Vlterbi decoder, and is operative to detect the read sampling value sequence as successive time series and to decode maximum likelihood binary reproduction data based on the detected time series.
In the recording information reproducing apparatus as mentioned above, the clock signal whose phase is synchronized with that of the read signal read from the recording disc 3 is generated by the PLL circuit 10 and the read signal is sampled by the clock signal, thereby obtaining the read sampling value sequence.
If, however, a linear recording density of the information recorded on the recording disc 3 is high, the read signal will be influenced by a rotation variation of the spindle motor 2, an eccentricity of the recording disc 3 itself, or the like and a time base fluctuation will occur in the read signal. In such a case, if the PLL circuit 10 becomes unable to follow the time base fluctuation, a phase deviation develops in the read sampling value sequence generated from the A/D converter 5. Therefore, residual errors may remain in the read sampling value sequence due to the influence of the phase deviation and a problem arises such that the decoding circuit 30 at a later stage cannot perform a very reliable decoding operation.